In today's printers, a document cannot be printed exactly the same over repeated printing, for example, hundreds of thousands of pages. Color quality will not be the same. For example, the shade of red will not be printed with the same shade from page 1 to page 1 million. Over time, the shade of red will look more “pinkish” in color. In order for the printer to print the same shade of red, the printer has to go through “color calibration”. The concept is that when the printer prints the page looking “pinkish”, the controller will compensate for the lack of redness and make the red color more “red”, so that when the printer prints the page, it will lighten up the red.
For example, the color RED can be produced by mixing 100% of Yellow, 99% of Magenta, 0% of Cyan and 0% of Black. If for some reason the printed page looks pinkish, then the controller has to adjust the document so that it can increase Magenta to 100% and reduce Yellow to 70%. This way, the controller compensates the lack of “redness” by the printer by increasing the red component of the document. This process is called “calibration”.
To achieve color calibration, a four-step process can be performed by printing a document from a printer. In calibration, this document is a hardcoded, canned document called a measurement chart. A device called a sensor scans the measurement chart and measures the color density. Sensor can include a handheld version and an in-line version. The handheld requires manual/human intervention as the user has to pick up the printed measurement chart from the printer tray, scans the document with a handheld device. The in-line version does not require manual/human intervention as the printer can print the measurement chart. The in-line sensor can be located on a paper path the output tray), and can measure the color density of the chart as the color chart exits the printer. The color measurements from the sensor are then sent to the controller. The controller adjusts the colors of the document to compensate for the printer's lack of color. The measurement chart can be resent again with the adjusted colors.
However, no two printers will print the same document exactly alike, even if the printers are exactly the same model since every printer has its own unique level of wear and tear. Over time, color quality will diminish over repeated printing, which can be become a problem with print shops because there may be situations where the print shop will receive a large print job order that requires multiple printers to finish the job within a tight schedule. In addition, the print shop may have a problem if the same document is split to different printers and each copy of the document is printed differently because color quality is different amongst the printers.
Thus, to perform color calibration “in-line”, each printer must have an in-line sensor. In addition, it can be difficult to ensure that each printer will print the document the same, regardless of the differences of each printer's wear and tear.